1. Field of the Invention
The present invention relates to measurement method for injection molding machines in which measurement is performed by rotating a screw with an installed back-flow valve, and the back-flow prevention valve is forcibly closed.
2. Description of the Relevant Art
In general, in inline screw type injection molding machines, back-flow of molten resin during injection is prevented by installing a back-flow prevention valve (ring valve) on the tip of a screw. In this kind of injection molding machine, measurement method is conventionally performed in order to provide more reliable back-flow prevention, in which the back-flow prevention valve is forcibly closed by rotating the screw in reverse by a small number of rotations (rotation angle), after performing measurement by rotating the screw (with forward rotation).
For example, disclosed in Japanese Unexamined Patent Publication No. 11 (1999)-240052 is an operation method (measurement method) of an inline type injection molding machine in which the amount of measured molten mixed resin to be injected is prevented from changing during suck-back and injection, reducing dispersion in the amount of fill. In this operation method, after measuring molten mixed resin pooled in the tip of a heating tube, a screw is rotated and a check ring (ring valve) is moved until it makes contact with a check sheet to close a resin path formed between the check ring and the check sheet, and then suck-back processing is performed.
However, the following problems exist in the conventional operation method (measurement method) of injection molding machines.
First, because the reverse rotation of the screw is controlled by a set time or amount of screw rotation, dispersion easily occurs in the position of the back-flow prevention valve (ring valve) or resin pressure after reverse rotation. Finally, it becomes insufficient to prevent variation in the amount of measured resin between shots, making it impossible to secure high measurement accuracy.
Second, because the screw is rotated in reverse after measurement is over when resin pressure in the rear of the back-flow prevention valve becomes higher than in the front, it is difficult for the back-flow prevention valve to move backward (closing direction), making it impossible to close the back-flow prevention valve fully. Even if suck-back processing which makes the screw retreat is subsequently performed, the closed back-flow prevention valve may open again, making it impossible to close the back-flow prevention valve securely and stably before starting injection.